Volunteers monitor under the Virginia Save Our Streams (VASOS) protocols to determine the health of their streams by counting, sorting, and identifying macroinvertebrates. 

Per the VASOS webpage:

"Aquatic benthic macroinvertebrate: an organism without a backbone that lives underwater at the bottom of the habitat and can be seen by the human eye.

The organisms VA SOS volunteers identify live in the bottom of freshwater streams. Some of these organisms are larvae, and will spend their adult lives on land or in the air. Others spend their entire lives underwater. Many cling to rocks and cobbles at the bottom of the stream and catch food as it flows by, while others prowl the habitat looking for prey. All of these 'macros; will spend at least a few months living in the river or stream.

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The streams and creeks these organisms call home can change dramatically over the course of years, months, weeks, and even days. A sudden heat wave can quickly heat a stream; a rainstorm wash fertilizer and road salt into a creek; an oil spill fill the water with toxins. Some changes occur naturally with the seasons, but as more and more homes, roads, and businesses are built along streams, artificial changes will start to pollute and stress the habitat."

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Source: Izaak Walton League Virginia Save our Streams